Simple blackjack

Simple blackjack

Here's the question: Given 2 int values greater than 0, return whichever
value is nearest to 21 without going over. Return 0 if they both go over.
blackjack(19, 21) ¨ 21
blackjack(21, 19) ¨ 21
blackjack(19, 22) ¨ 19
What I have so far:
public int blackjack(int a, int b) {
if (a>21 && b>21){
return 0;
}
if (a<21 && b>21){
return a;
}
if (b<21 && a>21){
return b;
}
if (21-a < 21-b){
return a;
}
return b;
}
This question is from codingbat.com, and for all the tests that it shows,
this code works, but when it finishes and displays "other tests", this
code fails. I suppose there's a certain situation where this wouldn't
work, but I can't think of it right now. Any thoughts?